Zeolites are very common materials in nature and there are many types of synthetic zeolites. It is estimated that there are 34 species of zeolite minerals and about 100 types of synthetic zeolites.
Zeolites are used in a wide range of chemical process technologies. The wide variety of applications includes separation and recovery of normal paraffin hydrocarbons, catalyst for hydrocarbon reactions, drying of refrigerants, separation of air components, carrying catalyst in the curing of plastics and rubber, recovering radioactive ions from radioactive waste solutions, removing carbon dioxide at high altitudes, solubilizing enzymes, separating hydrogen isotopes, and removal of atmospheric pollutants such as sulfur dioxide. Cracking catalysts, such as those used in fluid catalytic cracking (FCC) and hydrocracking of hydrocarbon fractions, contain crystalline zeolites, often referred to as molecular sieves, and are now used in almost 100% of the FCC units, which process about 10 million barrels of oil per day.
Zeolites, or molecular sieves, have pores of uniform size, typically ranging from 3 to 10 angstroms, which are uniquely determined by the unit structure of the crystal. These pores will completely exclude molecules which are larger than the pore diameter. As formed in nature or synthesized, zeolites are crystalline, hydrated aluminosilicates of the Group I and Group II elements, in particular, sodium, potassium, magnesium, calcium, strontium, and barium, which can be exchanged with higher polyvalent ions, such as rare earths or with hydrogen. Structurally, the zeolites are "framework" aluminosilicates which are based on an infinitely extending three-dimensional network of AlO.sub.4 and SiO.sub.4 tetrahedra linked to each other by sharing all of the oxygens. Zeolites may be represented by the empirical formula: EQU M.sub.2/n O.multidot.Al.sub.2 O.sub.3 .multidot.xSiO.sub.2 .multidot.yH.sub.2 O
In this oxide formula, x is generally equal to or greater than 2 since AlO.sub.4 tetrahedra are joined only to SiO.sub.4 tetrahedra, n is the cation valence. The framework contains channels and interconnected voids which are occupied by the cation and water molecules. The cations are quite mobile and may be exchanged, to varying degrees, by other cations. Intercrystalline "zeolitic" water in many zeolites is removed continuously and reversibly. In many other zeolites, mineral and synthetic cation exchange or dehydration may produce structural changes in the framework.
As stated above, the uses for zeolites are many, but they typically must be combined with other materials when they are used in process applications. As an example, a synthesized zeolitic material, which is usually less than 4 microns in size, is combined with a binding agent, such as kaolin clay, silica sol, or amorphous silica, alumina, and zirconia as described in Demmel's U.S. Pat. No. 4,826,793 and then spray dried or extruded to produce a finished material that has the properties desired for the intended use. These properties may include attrition resistance, crush strength, particle size distribution, surface area, matrix area, activity and stability. Another method of producing a finished zeolite containing product would be to produce the zeolite in-situ as described in Hayden's U.S. Pat. No. 3,647,718. While these patents deal mainly with FCC type catalyst, similar procedures are used in the production of zeolitic materials for other process applications. As an example, most fixed bed zeolytic catalyst, such as those used in hydrocracking, alkylation, dealkylation, transalkylation, isomerization, polymerization, and separation processes, disperse the zeolytic component in a pellet that consists mainly of alumina. Based on our discovery it is our belief that in the manufacture of these fixed bed pelleted zeolitic catalyst and zeolitic FCC type catalyst that some of the zeolitic pores are blocked or buried within the matrix material and that our process can remove this blockage and increase the available zeolite. So not only is our process applicable to spent or equilibrium catalyst, but also to fresh catalyst.
An objective in refining crude petroleum oil has always been to produce maximum quantities of the highest value added products in order to improve the profitability of the refining. Except for specialty products with limited markets, the highest value added products of oil refining with the largest market have been transportation fuels, such as gasoline, jet fuel and diesel fuels. Historically, a major problem in the refining of crude oil has been to maximize the production of transportation fuels. This requires a refining process or method which can economically convert the heavy residual oil, the crude oil fraction boiling above about 1000.degree. F., into the lighter boiling range transportation fuels. A major obstacle to the processing of this heavy residual oil has been the concentration of refining catalyst poisons, such as metals, nitrogen, sulfur, and asphaltenes (coke precursors), in this portion of the crude oil.
Since most of the oil refineries in the world use the well known fluid catalytic cracking (FCC) process as the major process for the upgrading of heavy gas oils to transportation fuels, it is only natural that the FCC process should be considered for use in the processing of heavy residual oils. Indeed, this has been the case for the last ten to fifteen years. However, the amount of residual oil that a refiner has been able to economically convert in the FCC process has been limited by the cost of replacement catalyst required as a result of catalyst deactivation which results from the metals in the feedstock. The buildup of other catalyst poisons on the catalyst, such as the coke precursors, nitrogen and sulfur, can be effectively controlled by using catalyst coolers to negate the effect of coke formation from the asphaltene compounds, using regenerator flue gas treating to negate the environmental effects of feed sulfur, and using a short contact time FCC process, such as that described in U.S. Pat. No. 4,985,136, to negate the effects of feed nitrogen, and to some degree, the feed metals.
For the past twenty or more years the most widely used FCC catalysts have been zeolitic catalysts, which are finely divided particles formed of a relatively inert matrix, usually silica-alumina, alumina or the like, having a highly active zeolitic material dispersed in the matrix. As is well-known, the zeolites used in such catalysts are crystalline and typically have a structure of interconnecting pores having a pore size selected to permit the ingress of the hydrocarbon molecules to be converted, and the zeolite has a very high cracking activity. Therefore, the highly active zeolite is dispersed in a matrix having a lesser cracking activity in a ratio providing the desired activity for commercial use. Typically used zeolites are of the faujasitic type, e.g., X-, Y- or L-type synthetic zeolites, and from about 5 wt. % to about 70 wt. % of the zeolite is employed. Such zeolitic FCC catalysts, their manufacture and their use in the FCC process are well known by those working in the art.
It is commonly accepted in the oil refining industry that vanadium contained in the residual oil FCC feedstock will irreversibly deactivate the zeolite by attacking the structure, and that this vanadium effect is more pronounced at temperatures above about 1330.degree. F. It is also commonly accepted that catalyst deactivation by hydrothermal deactivation or by metals (e.g., sodium and vanadium) attack is irreversible.
In the operation of an FCC process unit (FCU) the process economics are highly dependant upon the replacement rate of the circulating catalyst (equilibrium catalyst) with fresh catalyst. Equilibrium catalyst is FCC catalyst which has been circulated in the FCC between the reactor and regenerator over a number of cycles. The amount of fresh catalyst addition required, or the catalyst replacement rate, is determined by the catalyst loss rate and that rate necessary to maintain the desired equilibrium catalyst activity and selectivity to produce the optimum yield structure. In the case of operations wherein a feedstock containing residual oil is employed, it is also necessary to add sufficient replacement catalyst to maintain the metals level on the circulating catalyst at a level below which the FCC yield structure is still economically viable. In many cases, low metal equilibrium catalyst with good activity is added along with fresh catalyst to maintain the proper FCC catalyst balance at the lowest cost.
In the processing applications that utilize zeolites, the material must be replaced as it looses its ability to perform the desired function. That is, the zeolitic material deactivates under the conditions employed in the process. In some cases, such as FCC and TCC type catalytic applications, fresh zeolitic material, in this case zeolitic catalyst or additives such as ZSM-5 (described in U.S. Pat. No. 3,703,886), are added on a daily basis. Fresh zeolitic catalyst is added daily at a typical rate of from 1% to as high as 10% of the process unit inventory to maintain the desired activity in the plant. Other zeolitic catalysts, such as those used in hydrocracking, alkylation, dealkylation, transalkylation, isomerization, polymerization, and separation processes, are usually replaced as a batch when the zeolitic material deactivates to a certain point, at which the plant is shutdown and the zeolite replaced.
As will be seen from the following discussion, it is our belief that many types of zeolitic catalyst can benefit from the present invention, because, contrary to popular belief, the major cause of zeolitic catalyst activity decline is zeolite pore blockage which can occur, even during the catalyst manufacturing process, due to free silica or alumina, or compounds of silica or alumina, or other materials which are left behind and block the zeolite pore openings.
A primary object of the present invention is to enable the removal of zeolitic catalyst deactivating materials without destroying the integrity of the catalyst and, at the same time, to significantly improve the activity and selectivity of the catalyst. Another object of the present process is to reactivate zeolite-containing equilibrium catalyst using an environmentally safe and acceptable process. Still another object of the present invention is to improve the activity of various types of zeolitic catalyst and other zeolite-containing particulate solids, especially those that deactivate during use in the processing of hydrocarbons.
A further object of the present invention is improve the FCC equilibrium catalyst activity and selectivity. Another object of this invention is to improve the activity of fresh zeolitic catalyst. Still another objective of the invention is to reduce the requirement for fresh catalyst replacement to an FCC unit, which will reduce fresh catalyst costs, transportation costs, equilibrium catalyst disposal costs, and unit catalyst losses. Other objects of the invention will become apparent from the following description and/or practice of the invention.